Years of planning went into this monument, which opened on the tenth anniversary of the September 11 attacks. Called "Reflecting Absence," the design by architect Michael Arad features two reflecting pools that sit within the footprint of each of the Twin Towers. The water elements are surrounded by oak trees, and the placement of that plant life is not random. "The first ring of trees around each pool is exactly 212 feet across, from one side to the other, and that's the dimension of the towers," explains Arad. "All of the structural steel columns [of the Twin Towers] were on the perimeter of the building, and where those columns used to be, we have these trees growing." Arad believes that the timbers function as a living memorial to the fallen buildings. "It's a nice way of demarcating the tower footprints without being very literal about it," he says. Enter at Albany and Greenwich Sts (212-312-8800, 911memorial.org)